1. Field of the Invention
This invention relates to the navigation of an aircraft (fixed wing or helicopter) relative to ships at sea.
More specifically, it relates to the takeoff, landing and sling load re-supply operations involving a helicopter and a sea-going vessel.
2. Background Information
Shipboard helicopter operations provide a difficult operational environment. Wind over the deck and wake turbulence shed by the ship super structure offer challenging and unpredictable conditions during takeoff and landing. This is especially true in the operational environment that includes sea-state six, with associated twenty-foot and thirty-three knot winds. Anything other than calm seas can create pitch, roll, yaw and heave of the landing platform. Different sea-going vessels behave in a variety of ways due to their size, hull design, stabilization systems, etc. Of particular concern in this environment is the performance consistency of helicopter operations involving ship-based platforms. A helicopter pilot operating off such a platform must observe the heave, pitch, and roll motion of the landing platform and determine the landing contact time based on and roll motion of the landing platform and determine the landing contact time based on human reaction time as well as aircraft performance. The invention is directed to this difficult task.
Landing approaches are made either from directly behind the moving platform or from an angle off to the left or right, generally 45 degrees to the direction of ship travel. The aircraft heading will either be adjusted to be that of the ship, or will remain at a 45 degree angle to the ship heading to avoid pointing weapons at the ship super structure. Maneuvering the aircraft over the heli-deck from behind the ship involves a forward cyclic control input and appropriate directional control inputs to maintain desired heading. If the aircraft has been pre-positioned to the side of the ship, a lateral cyclic control input is made to effect a side-step motion, with directional control inputs as necessary to maintain desired heading. In either case, the collective control is adjusted to accommodate the power requirement to maintain altitude. For a ship having a nominal 25 foot deck height above water, transitioning from a hover 35 feet above water to 10 feet above a solid deck surface produces a noticeable change in hover power required. During heaving sea conditions with the landing deck moving several feet up and down, the helicopter must pick a power setting that will maintain a safe distance above the landing deck, while accepting some variation in true altitude relative to the landing deck, rather than constantly modulate collective power in an effort to always maintain a constant height above deck altitude.
Helicopter landing approaches to moving ships are performed in various ways but have several elements in common. First the helicopter makes an approach to a point in space either behind or adjacent to the heli-deck, coming to a stabilized hover approximately 2 main rotor diameters away from the edge of the platforms, and perhaps 10 feet above the platform at its highest heave elevation. Once the helicopter is in a stable hover condition (essentially formation flight with the vessel to be landed on), the motion of the landing deck is evaluated to determine an adequate period of quiescence, during which a safe landing can be made. As this motion is evaluated, the helicopter is carefully maneuvered into a position above the center of the landing deck, maintaining an average position of perhaps 10 feet above the deck. When the decision is made to land, the horizontal position over the heli-deck is maintained, and power is reduced to facilitate a rapid, firm landing during the period of quiescence, which may last as little as 5 seconds. Upon touchdown, systems such as a Harpoon device are employed to firmly clamp the helicopter to the heli-deck.